TODO

  o  add validation of XPath (lower prio) (when, must)

  o  add more stuff to XSD output:
     o  use substitutiongroup for augment

  o  add validation of instance-identifier defaults

  o  move all plugin API functions to plugin.py

  o  give a warning if the default case does not have any default leafs

Optimizations:

  o  lazy read imported modules.  do not validate the module on import,
     but do it when something is used.  even better would be to lazy
     validate.  hmm, maybe separate the side-effect free validation
     functions from the functions that have side-effect (like expand).